Gliese 581g: A Potentially Habitable World or Not?

"No matter what they say, it's just really hard to find a planet that's in this habitable zone that also has water, clouds, and is small enough that it doesn't have a lot of gravity," said St. Louis University's Paul Czysz. "There's about 100 million to a billion stars in our galaxy. ... It's like you're in the Atlantic ocean and [looking] for fish within just a six-foot circle of your boat."

There's been considerable debate over the existence of
Gliese 581g ever since the discovery of the
"Goldilocks" planet was
first reported nearly two years ago, but new research claims to provide additional evidence that the potentially habitable "super-Earth" really is out there.

The orbits of planets in the Gliese 581 system compared to those of our own solar system. The Gliese 581 star has about 30 percent the mass of our Sun, and the outermost planet is closer to its star than the Earth is to the Sun. The 4th planet, G, is a planet that could sustain life. (Credit: Zina Deretsky, National Science Foundation)

The prospect of the extrasolar planet, which is said to orbit the red dwarf star Gliese 581, 22 light-years from Earth, is an exciting one. It's thought to lie in the star's "habitable zone," where liquid water could exist on the planet's surface. That, in turn, could mean that it might be able to support life.

"Our findings offer a very compelling case for a potentially habitable planet," said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz, at the time of the discovery in late September 2010. "The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common."

'Liquid Water Is a Distinct Possibility'

Two weeks later, however, the controversy began when a European team declared that they could find
no evidence of the planet in their own examination of the same planetary system.

Instead of six planets orbiting Gliese 581, the European researchers saw evidence only of the four that had previously been found, they said.

In this latest development, a new, expanded analysis led once again by Vogt generated results that the American researchers say do indeed support the existence of Gliese 581g.

"This signal has a False Alarm Probability of < 4 percent and is consistent with a planet ... orbiting squarely in the star's Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility," concludes the new paper, which was
published online earlier this month.

The question now is, which of these two accounts gets the story right?

'These Are All Experienced Observers'

"It is extremely difficult for an outsider to assess which of the two teams is correct," Mario Livio, a senior astrophysicist with the
Space Telescope Science Institute, told TechNewsWorld.

"These are all experienced observers," Livio added. "The controversy, more that anything else, simply demonstrates that the analysis of the data is extremely difficult."

Indeed, "the question is, is it there because the analysis says it is, or because they ran enough options that they finally got the solution they were looking for?" wondered Paul Czysz, professor emeritus of aerospace engineering with
St. Louis University.

'Wiggles in the Light'

"When you're looking that far away, and inferring all kinds of orbits, and all you basically see are wiggles in the light coming from the sun," it's difficult to know what's actually being observed, Czysz told TechNewsWorld.

Essentially, the researchers assumed that "this little spike is a planet going in front of the sun, and then threw that into an analysis program to see what they get," he added.

Putting it more precisely, "the data set is the observed Doppler shifts of the star light that result from a component of the star's motion due to the gravitation pulls of multiple objects in orbit around the star," Scott Austin, associate professor of astronomy and director of the astronomical facilities at the
University of Central Arkansas, told TechNewsWorld.

'There May Be Multiple Solutions Found'

"The goal is to create a computer model of this multi-body gravitational system that can recreate the observed wiggles in Doppler-shift versus time data," Austin explained.

However, "the more physics and physical parameters one includes in the model, the longer it takes to find a match to the data; there may also be multiple solutions found," he pointed out.

Complicating the situation further, the data itself has a level of uncertainty "due to the signal-to-noise ratio of the spectra," Austin added. "How often the system was observed and over what length of time can also come into play."

In any case, the bottom line is that "some groups are making assumptions and simplifications to their model in order to speed up the solution search," he said.

The question then becomes, are those assumptions and simplifications valid, and do they produce a gravitationally stable system?

"The more complete modeling seems to suggest that the planet is there, but then the issue is proving that it is an actual detection versus unintentionally fitting the noise in the data," Austin concluded.

'It's Just Really Hard'

If the Vogt team's assumptions are correct, "it's possible there is a planet in this preferred zone," Czysz conceded. "The difficulty is, there really aren't very many planetary systems that have a good habitable zone.

"No matter what they say, it's just really hard to find a planet that's in this habitable zone that also has water, clouds, and is small enough that it doesn't have a lot of gravity," he pointed out.

Given the vast size of our galaxy, the odds are clearly against us.

"There's about 100 million to a billion stars in our galaxy," Czysz said. Potentially habitable planets are surely out there, "but it's like you're in the Atlantic ocean and [looking] for fish within just a six-foot circle of your boat."

'We're Still Babies Crawling on the Floor'

We also simply don't have the capability yet to go out and find such planets, Czysz added.

Crossing our galaxy so as to avoid its central supermassive black hole, for example, would take some 50,000 years, he pointed out.

"We need to figure out how
Jean-Luc did it," Czysz said. "We know there is a way to do it -- the difficulty is that we'd have no way of understanding how one implements that. We're still babies crawling on the floor."

'More Planets Are Being Discovered'

As for Gliese 581g, "this probably isn't going to be sorted out until it is possible to get sufficiently high enough signal-to-noise data," Austin suggested.

At the same time, "given that many more extrasolar planets are being discovered now, the expectation is that quite a number of them will be in the habitable zones of their host stars," Livio pointed out. "This will make the question of whether planet Gliese 581g exists or not less critical."

Of course, if and when we do find such planets in habitable zones, there won't be any rush to begin packing our bags.

"There is a tendency for the public to come away with the impression that more is being claimed than what is actually being claimed," Austin warned. "'Planet in the habitable zone' does not equal 'a planet that is habitable by life found on the Earth.'"

Katherine Noyes has been covering business and technology for more decades than she cares to admit, but all those years watching "Star Trek" and "Dr. Who" left too big an impression to ignore. You can find her on Google+ and
Twitter.